This invention relates to electronic impedance converter circuits and more particularly to impedance invertors, or gyrators, and even more particularly to phase-compensated gyrators and integrators.
One of the advantages of electronic integrated circuits (ICs) is small size, and so ICs have become ubiquitous in hand-held and other equipment, but some circuits require components that are difficult to integrate. In particular, it is difficult to integrate passive inductors having impedances of more than a few nanohenries (nH). Thus for example, on-chip passive signal filters are normally limited to resistor-capacitor (RC) filters, except for filters designed for very high frequencies where inductors (coils) of a few nH are effective.
In communications and electronics, a filter is generally hardware or software that selectively passes certain elements of a signal and eliminates or minimizes others. A filter in a telecommunication network, for example, may transmit signal elements either up to a certain frequency and attenuate (dampen) those above it (a low-pass filter), or down to a certain frequency and attenuate those below it (a high-pass filter), or within a band of frequencies (a band-pass filter).
It is possible to overcome the limited impedances of integrable passive inductors by using combinations of active devices (e.g., op-amps), resistors, and capacitors that are easily integrable. One combination that can mimic the properties of a passive inductor is the integrator, which can generally be considered to convert an impedance to its inverse. Integrators are often used in discrete-time (digital) filters. Continuous-time (analog) filters implemented with integrators typically employ such elements in loops and these loops are often connected back-to-back. Two integrators in a loop actually form a gyrator, and if the forward and backward integrators have the same transfer characteristics they form a passive gyrator, otherwise they form an active (or asymmetric) gyrator.
Gyrators are described in the literature, which includes P. Horowitz et al., The Art of Electronics 2d ed., pp. 266-267, 281 Cambridge University Press (1989); Fink et al., Electronic Engineer""s Handbook 4th ed. (D. Christiansen et al. eds.), pp. 16.38-16.39 McGraw-Hill, New York (1997); and B. Nauta, xe2x80x9cA CMOS Transconductance-C Filter Technique for Very High Frequenciesxe2x80x9d, IEEE J. Solid-State Circuits, vol. 27, no.2, pp. 142-153 (February 1992). Various circuits employing gyrators and integrators are described in German Patent Application No. 199 364 30-3 filed on Aug. 3, 1999, by S. Mattisson for xe2x80x9cAnalog Filterxe2x80x9d; and U.S. patent application Ser. No. 09/274,327 filed on Mar. 23, 1999, by S. Mattisson for xe2x80x9cDemodulator Circuitsxe2x80x9d, both of which are expressly incorporated in this application by reference.
In general, the output signal produced by a gyrator is delayed in time and is shifted in phase with respect to the input signal provided to the gyrator. As seen from FIGS. 1A, B, a gyrator generally comprises a positive transconductance gm and a negative transconductance xe2x88x92gm that receive input voltage signals v1, v2 and produce a combined output current signal.
Gyrators can be implemented in various circuits. For example, a gyrator can be implemented in a combination of inverter circuits as illustrated in FIG. 2, which depicts a gyrator core 10 having four invertors 12, 14, 16, 18 arranged as described in the above-cited Nauta publication. The negative transconductance is realized by employing differential input signals i_1, i_2 and differential output signals o_1, o_2 and by crossing one pair of output-to-input connections. This crossing produces a loop through the four invertors (transconductances), and it will be appreciated that although FIG. 2 shows the connection between inverter 12""s output and inverter 16""s input crossing the connection between inverter 18""s output and inverter 14""s input, it is possible for the loop to be formed by crossing the connection between the inverter 12""s input and the inverter 16""s output and the connection between the inverter 14""s output and the inverter 18""s input.
Such a filter typically may also include sections 20, 30 for feeding back common-mode voltages. As depicted in FIG. 2, section 20 has four invertors 22, 24, 26, 28 and section 30 has four invertors 32, 34, 36, 38.
Crossing connections as depicted in FIG. 2 can cause stability problems in circuits using such gyrator cores. It will be appreciated that a stability analysis of a gyrator-based filter such as that depicted in FIG. 2 is substantially the same as the stability analysis of an integrator-based filter because integrators are parts of gyrator loops. First-order stability analysis such as that described in the Nauta publication reveals that stable, i.e., non-regenerative, behavior can be obtained only if invertors are used in the gyrator core 10. For example, the transconductances, e.g., invertors 12, 14, 16, 18, may be single metal-oxide-semiconductor (MOS) transistors, including complementary MOS (CMOS) field-effect transistors (FETs). Using MOS components, and in particular CMOS components, has a number of advantages, not the least of which is low power consumption.
More detailed analysis of the MOS transistors, however, shows that non-quasi-static behavior of channel charges in each MOS device adds a delay that can be approximated by a parasitic pole in the frequency response of the transconductance of each device. This extra pole, or delay, makes the gyrator unstable if it is not designed properly. The actual performance of the simple filters described in the Nauta publication generally accords with Nauta""s predictions, but the actual performance of the complicated filters described by Nauta may deviate by as much as 10 dB from the predicted behaviors.
Nauta""s filters could be stabilized by separate Q-tuning circuits (e.g., a separate supply voltage for ballast invertors in common-mode feedback networks) that could be externally adjusted, but then the filter""s transfer characteristic was altered significantly from that predicted because the gyrator""s Q does not depend only on the output conductances of the gyrator transistors but also on the channel delay and the external resonator (loading capacitance). Lower-order filters might work because the external terminations would provide sufficient loading of the gyrators to make them stable, but higher-order filters would have internal nodes that did not get sufficient loading to make the filters stable, unless the resonance frequencies were very low compared to the transit frequencies fT of the devices in the gyrator cores or every gyrator were given a separate Q-tuning circuit, which would be impractical.
German Patent Application No. 199 580 96-0 filed on Dec. 2, 1999, by S. Mattisson for xe2x80x9cHigh-Q Gyrator Structuresxe2x80x9d, describes a method for enhancing the quality factor of gyrator resonators by balancing phase-lag and phase-lead terms in the gyrator structure. This works very well for parallel resonators having comparatively narrow bandwidths that emulate parallel LC resonators. Nevertheless, it is desirable for general-purpose impedance invertors to have wide bandwidths, and the job of balancing terms is complicated by the capacitances present at the gyrator terminals. In general, it is difficult completely to xe2x80x9cabsorbxe2x80x9d those capacitances in the impedances connected to the gyrator without violating other design constraints, ultimately resulting in the gyrator""s having excess phase lag. Such phase lag in a wide bandwidth gyrator manifests itself as a signal loss, with the result that the total insertion loss of the filter an become excessive. This patent application is expressly incorporated in this application by reference.
Patent Application No. 9916808.0 filed in the United Kingdom on Jul. 16, 1999, by S. Mattisson for xe2x80x9cIntegrated Circuitxe2x80x9d describes a method of widening the useful frequency bandwidth of a filter based on Nauta gyrators or invertors by taking into account the channel delays of MOS devices. The patent describes that it is the phase lag of the gyrator-core transadmittances (see FIG. 2) that ruins the gyrator stability. Although the transadmittance ym of a MOS device is normally assumed to be purely conductive, which would result in a stable system matrix, a filter such as that depicted in FIG. 2 is often unstable in practice due to non-quasi-static channel delays. This patent application is also expressly incorporated in this application by reference.
Furthermore, Applicant believes no complex active high-frequency continuous-time on-chip MOS filter has been successfully fabricated due to the unreliability of the gyrator cell. Successful filters have been simple, limited to lower-order filters or cascades of low-order filters (with inferior sensitivity characteristics) or more complex filters at low frequencies. It is thus desirable to devise a method for phase compensating a gyrator over a wide frequency bandwidth and to provide circuits enjoying such phase compensation.
Applicant""s invention overcomes these limitations of the prior art and enables gyrators and similar devices to be phase compensated over a wide frequency bandwidth.
In one aspect of Applicant""s invention, an integrated circuit gyrator has series feedback associated with at least some of the transistors in the gyrator, and the series feedback is provided by at least one stack of a plurality of feedback transistors that are individually sized and capacitively compensated so as to compensate effects of non-quasi-static delays of transistors in the gyrator. The gyrator may include a plurality of invertors that are arranged in a loop and that are cross-connected, and the transistors in the gyrator and the feedback transistors may be metal-oxide-semiconductor devices, bipolar semiconductor devices, or bipolar complementary metal-oxide-semiconductor devices. The stack of feedback transistors may operate according to class AB and provide phase-lead compensation.
In another aspect of Applicant""s invention, an integrated circuit gyrator has series feedback associated with at least some of the semiconductor devices in the gyrator, and the series feedback is provided by at least one stack of feedback networks that are individually sized and capacitively compensated so as to compensate effects of non-quasi-static delays of semiconductor devices in the gyrator. The gyrator may include a plurality of invertors that are arranged in a loop and that are cross-connected, and the semiconductor devices in the gyrator may be metal-oxide-semiconductor devices, bipolar semiconductor devices, or bipolar complementary metal-oxide-semiconductor devices. The stack of feedback networks may include a plurality of parallel combinations of a resistor and a capacitor.
In another aspect of Applicant""s invention, an electronic signal filter has a core that includes a plurality of metal-oxide-semiconductor (MOS) devices cross-connected in a loop, and series feedback stacks respectively connected to the MOS devices. Each stack includes at least two MOS devices that have parameters selected such that phase lag due to non-quasi-static channel delays of the MOS devices in the core is reduced.